Proto Melange

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Lithium isotopes in Guatemalan and Franciscan HP-LT Rocks: Insights into the role of sediment-derived fluids in the mantle wedge. Kyla Simons, George Harlow, Hannes Brueckner , Steven Goldstein, Sorena Sorensen, N. Gary Hemming, Charles Langmuir. Proto Melange. Oceanic Sediments. - PowerPoint PPT Presentation

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  • Lithium isotopes in Guatemalan and Franciscan HP-LT Rocks: Insights into the role of sediment-derived fluids in the mantle wedge

  • Proto MelangeShear zone between platesAltered Ocean CrustOceanic SedimentsAfter Bebout (2007)

  • Subduction Fluid Trace Element Patterns

  • HP-LT Rocks in Guatemala

  • Dehydration of SlabShear zone between platesAltered Ocean CrustOceanic SedimentsPhysical Model Prograde

  • Physical Model RetrogradeDehydration of SlabShear zone between platesAltered Ocean CrustOceanic Sediments

  • South have lawsonite cold and wet @ peak conditions (~480C)Very high pressure rocks just shy of coesite (UHP) fieldNorth saw slightly higher-T (
  • Li systematics in GuatemalaMORBIABGuatemala Eclogite Avg.

    ****mention the age of rocks to be ~130Ma, therefore ancient subduction and rocks north and south juxtaposed currently

    This is a close-up view of the study area. Serpentinite bodies are in green, while jadeitite are in dark blue, amphibolite in purple, eclogites in orange and blueschist (south of the fault) in light blue. Note that pristine eclogites are only found on the south side. Some of the amphibolites are thought to have been eclogites, but were recrytallized during a second higher-T metamorphic event.

    ***The prograde P-T path for the eclogites shows that were cold and wet and reached P just why of coesite (UHP) field. Peak T is ~500C for southern eclogites based on (??), and slightly higher for northern eclogites that saw a second, later metamorphic event that the southern rocks did not.

    The jadeitites are METASOMATIC rocks, and therefore do not have a protolith. They generally follow the down-P gradient of the eclogites with higher pressures for southern rocks that contain qtz. Southern jadeitites have a range of T between 200-400C based on (??), while northern examples are slightly higher b/c of the later second metamorphic event. Overall, these rocks record a retrograde metasomatic signature from aqueous fluid alone, as these temperatures are too cold for melting. *This is a plot of Li isotopes vs. Li conc showing all the mineral separates and whole rocks measured for Guatemala. Note that Guatemala rocks are higher in Li conc and lighter in Li isotopes than the fields for MORB and IAB (taken from the lit). The average Guatemala eclogite both measured and calculated using model proportions of phases measured is between 30-40 ppm Li and ~ -1 permil +/- 2.*